Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States.
Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, United States.
J Phys Chem Lett. 2024 Sep 19;15(37):9456-9465. doi: 10.1021/acs.jpclett.4c01826. Epub 2024 Sep 9.
Vibrational-electronic (vibronic) coupling plays a critical role in excitation energy transfer in molecular aggregates and pigment-protein complexes (PPCs). But the interplay between excitonic delocalization and vibronic interactions is complex, often leaving even qualitative questions as to what conceptual framework (e.g., Redfield versus Förster theory) should be used to interpret experimental results. To shed light on this issue, we report here on the interplay between excitonic delocalization and vibronic coupling in site-directed mutants of the water-soluble chlorophyll protein (WSCP), as reflected in 77 K fluorescence spectra. Experimentally, we find that in PPCs where excitonic delocalization is disrupted (either by mutagenesis or heterodimer formation), the relative intensity of the vibrational sideband (VSB) in fluorescence spectra is suppressed by up to 37% compared to that of the native protein. Numerical simulations reveal that this effect results from the localization of high-frequency vibrations in the coupled system; while excitonic delocalization suppresses the purely electronic transition due to H-aggregate-like dipole-dipole interference, high-frequency vibrations are unaffected, leading to a relative enhancement of the VSB. By comparing VSB intensities of PPCs both in the presence and absence of excitonic delocalization, we extract a set of "local" Huang-Rhys (HR) factors for Chl in WSCP. More generally, our results suggest a significant role for geometric effects in controlling energy-transfer rates (which depend sensitively on absorption/fluorescence line shapes) in molecular aggregates and PPCs.
振动-电子（vibronic）耦合在分子聚集体和色素-蛋白复合物（PPCs）中的激发能量转移中起着关键作用。但是，激子离域和振动相互作用之间的相互作用很复杂，即使对于应该使用什么概念框架（例如，Redfield 理论与 Förster 理论）来解释实验结果，也常常存在定性问题。为了解决这个问题，我们在这里报告了在水溶性叶绿素蛋白（WSCP）的定点突变体中，激子离域与振动耦合之间的相互作用，这反映在 77 K 荧光光谱中。实验上，我们发现，在激子离域被破坏的 PPCs 中（无论是通过突变还是异二聚体形成），与天然蛋白质相比，荧光光谱中的振动边带（VSB）的相对强度被抑制了高达 37％。数值模拟表明，这种效应是由于耦合系统中高频振动的局域化所致；虽然激子离域由于类似于 H-聚集体的偶极-偶极干涉而抑制了纯电子跃迁，但高频振动不受影响，导致 VSB 相对增强。通过比较具有和不具有激子离域的 PPCs 的 VSB 强度，我们从 WSCP 中的 Chl 提取了一组“局部”Huang-Rhys（HR）因子。更一般地，我们的结果表明，几何效应在控制分子聚集体和 PPCs 中的能量转移速率（其对吸收/荧光线形状敏感）方面起着重要作用。
